Process and apparatus for fractional distillation



March 28,1944. M. R. FENSKE 2,344,984

PROCESS AND APPARATUS FOR FRACTIONAL DISTILLATION Filed July 17, 1940 2Sheets-Sheet 1 1 I l 19 Q k 1 2a M24 4 Y 25 I Z l/agor animal Y. ?O I pfiwenfir e210 safe a G rheu- V March 28,1944. F 'NSKE 2,344,984

PROCESS AND APPARATUS. FOR FRACTIONAL DISTILLATION Filed Jul'y 17, 19402 Sheets-Sheet 2 Patented Mar. 28, 1944 PROCESS AND APPARATUS FORFRACTIONAL DISTILLATION Merrell R. Fenske, State College, Pa., assignorto The Pennsylvania Research Corporation, a corporation of PennsylvaniaApplication July 17, 1940, Serial No. 345,884

8 Claims.

This application is a. continuation-in-part of my copending applicationSerial Number 157,925, filed August 7, 1937 which has matured intoPatent Number 2,208,573, dated July 23, 1940.

In fractional distillation, particularly as practiced on the plant scalewherein fractionating columns of considerable size are employed,considerable loss in through-put and efiiciency are occasioned by heatexchange with the atmosphere despite the application of heat insulationto exposed surfaces. These losses are due to the inability to cause thefractionating column to operate under conditions more nearly approachingadiabatic conditions.

This difliculty is also experienced in the laboratory when it is desiredto make close separations and has led to the expedient of wrappingcolumns with resistance wire for temperatur control through electricalcircuits when the fractionation takes place at temperatures above roomtemperature.

The opposite of this which is applicable when distillations take placeat temperatures below room temperature, such as in the fractionation ofliquid air,-is to replace the resistance wire with tubular coils for thecirculation of cooled brine or for the direct expansion of a suitablerefrigerant, such as takes place, for instance, in direct refrigerationprocesses.

In either case, that is, whether heat or cold is applied to the exteriorsurface of the fractionat ing column, the object is to hold thetemperature through control means approximately at temperatures alongthe column which would obtain if there were no heat exchange with thecolumn. The desideratum is to maintain the temperature gradient betweenthe column and its exterior at a virtual minimum.

Expedients of this character are for the most part limited to thelaboratory since their application to commercial types of columnsinvolves considerable expenditures in time, labor and materiats. Even inthe case of large laboratory columns, this is not a negligible item.

The cost of operation, particularly in the case of difficult separationsin commercial or semicommercial columns, might be very considerable.

In seeking to overcome the foregoing difiiculties, I have discoveredthat the heat gradient between a column and its exterior may be verysubstantially reduced if not actually brought to a virtual minimum bycausing the rectified vapors leaving the column and prior to theircondensation to enclose the fractionating zone of the column whilepassing on their way to the condenser.

By causing the rectified vapors to enclose the fractionating zone of thecolumn, heat exchange between the fractionating zone and its exterior isvery substantially reduced if not for practicable purpose eliminated.This procedure is particularly desirable and efficient when closeboiling components are being separated.

An outstanding advantage of my method when distillation takes place attemperatures above atmospheric is that any heat exchange between therectified vapors and the atmosphere has no substantial effect upon theheat gradient between the fractionating zone of the column and therectified vapors, since it is necessary to condense a considerable partor substantially all of the rectified vapors before any markedtemperatur reduction is possible. Thus the latent heat of vaporizationof .th rectified vapors is made available as a highly efiicient bufierto compensate for any heat exchange with the atmosphere without anysubstantial change in the temperature gradient between the fractionatingzone and its immediate surroundings.

While the heat of vaporization is not-made available for this purposewhen distillation takes place at sub-atmospheric temperatures, myprocess is nevertheless distinctly advantageous since the rectifiedvapors are capable of considerable quantitative loss or gain of sensiblheat without a material change in temperature because of theirconsiderable volume.

It will be noted that the vapor volume is independent of the refluxratio and in the case O adiabatic fractionation its determinedsubstantially solely by the heat input into the still-pot or reboiler. vp

Further features of the invention reside in the steps, combinations andsequences of steps, and in the construction, arrangement of andcombination of parts, all of which together With other features willbecome more apparent to persons skilled in the art upon becomingfamiliar herewith and upon reference to the drawings in which: g I

Figure 1 is a sectional elevation illustrating a conventionalfractionating column having my in} vention applied thereto; and

Figure 2 is a sectional elevation of a column of the multi-tubular typehaving my invention applied thereto.

Referring now more particularly to Figure 1, at I!) is shown a column ofwhich H is th fractionating zone. v

Fractionating zone H has beenillustrated without any interiorconstruction since this may take any conceivable form for thecountercurrent contact of an ascending vapor phase with a descondingliquid phase for the purpose of fractionation.

Engineering details have also been omitted since these are capable ofwide variation and will occur to persons skilled in the art uponbecoming familiar herewith.

Fractionating zone II, as illustrated, is surrounded by a shell I! ofsomewhat larger diameter thus forming a circumferential or annular spaceI 3 therebetween.

Shell I2 is shown provided with a dome or can M, the interior l5 ofwhich forms an open space above the top of fractionating zone I6 andcommunicates with space [3.

Extending downwardly into space I5 is a spray device I! for the deliveryof refiux liquid to fractionating zone H, spray I? being connected to apipe [8 which extends through dome l4 and connects to pipe l9 leadingfrom pump 20.

Pump is connected to the outlet 2! of condenser 22 the inlet 23 of whichis connected to opening 24 in shell l2 adjacent the bottom of thelatter.

A vapor phase feed and liquid phase withdrawal chamber 25 is connectedto the bottom of column l0 and communicates with the interior offractionating zone H.

In operation vapors to be fractionated leaving from any source enterchamber 25 and pass up into fractionating zone H where they arecountercurrently contacted with liquid reflux supplied through spray 7.

As previously pointed out the phase contacting means in fractionatin'gzone ll may take the form of any of thewide variety known in the art.

Common examples are the bubble cap construction and the packedarrangement. Further details as well as other examples may be had uponreferring to the vast amount of literature and patents on thisparticular subject.

, As a result of the countercurrent contact in fractionating zone II,the ascending vapors are rectified and upon rectification pass upwardlyout of the top l6 of fractionating zone II. The rectified vapors descendthrough annular space l3 about the outside of fractionating zone II,thus forming a heat exchange buffer between the fractionating column llandthe shell l2.

The rectified vapors then pass out through opening 24 into condenser 22in which, as illustrated, they are totally condensed.

Condensate from condenser 22 flows into metering pump 20 which returns apart through line l9 to spray H as reflux and withdraws another partwhich flows through line 21 as product.

Any condensate which might be formed in annular space l3 will collect inthe bottom thereof and fiow out through opening 24 into condenser 22 tobe added to the condensate formed in condenser 22.

While heat insulation may be added to the exterior of shell I 2, ifdesired, and likewise to the exterior of dome l4 and other parts of theapparatus, my invention makes it possible to reduce the extent of anysuch insulation, if desired, and; in many cases, to omit it entirelyWithout greatly increasing the heat gradient between the fractionatingzone and its immediate surroundings.

Since the latent-heat of vaporization must be removed from the rectifiedvapors descending through annular space l3 before the temperature inannular space I3 can be very materially reduced, it will be seen thatshell l2 might be made to function as an atmospheric temperaturecondenser thus relieving condenser 22 of a a part of the condensingload.

It Will be understood, of course, that liquid refiux after passing downthrough fractionating zone i 1 passes down through chamber 25 from whichit may be led to any desired point such as back to the still-pot orreboiler, or to another fractionating column.

Obviously fractionating zone ll may be operated under any desiredpressure for which purpose I have illustrated a vent 28 leading from thebottom of condenser 22, which may be open to the atmosphere in the eventof fractionation at atmospheric pressure, or may be connected topressure regulating mechanism for fractionating either atsub-atmospheric or super-atmospheric pressure.

The particular description of Figure 1 was given for the purposes ofillustration and it is to be understood that the invention may beapplied to any type of fractional distillation either for the purposesof enrichment or stripping, or otherwise, and may be embodied in anytype, form configuration, or construction of apparatus.

In some cases, for example, when the temperature difference between thetop and bottom of fractionating section II is relatively large, say ofthe order of or more, it may be desirable to provide section II withsome insulating means such as by applying heat insulating materials or adead-air space to the outside of section H in order to reduce heatexchange between the inside of II and the vapors in annular space l3.

Further adaptations of the invention will occur from time to time topersons skilled in the art upon becoming familiar herewith.

One such adaptation which is of particular interest because it in turninvolves a problem peculiar to the type of equipment involved, is theembodiment of the invention in a multi-tubular column.

This is illustrated in Figure 2 in which at 3B is shown a column havinga plurality of fractionating zones 3| arranged in parallel.

These fractionating zones have been illustrated without any interiorconstruction since this, as in the case of fractionating zone II, maytake any conceivable form for the counter-current contact of anascending vapor phase with a descending liquid phase for the purposes offractionation.

At present packing such as jack chain, Raschig rings, small wire forms,etc., are largely used for packing multi-tubular columns though as abovepointed out, any other material may be substituted.

Furthermore, any other type of fractionating unit might be employed,such as the bubble plate 7 construction, rotating mechanical devices,specialized forms of packing, etc.

For this reason engineering details have been omitted.

Fractionating zones 3!, as illustrated, are surrounded by a shell 32 ina manner to form a space between the fractionating zones and the shellas well as between the fractionating zones themselves. The aggregatespace will be referred to for convenience as space 33.

A can construction 34, which will be hereinafter more particularlydescribed, is shown at the top of shell 32 to close the space 33 at thetop, the construction being such as to leave the tops 36 offractionating zones 3| open to the space 33.. Cap 34 is designed tometer reflux liquid in proportionate quantities into the individualfracftionating zones 3| and is connected through pipe 39 toproportioning pump 43. Pump 40 is connected to the outlet 4| ofcondenser 42, the inlet 43 of which is connectedto opening 44 in shell32 adjacent the bottom of the latter. i

A vapor phase feed and liquid phase withdrawal chamber 45 is connectedto the bottom of column 30 and communicates with the fracticnating zones3| at their lower ends.

Returning now to cap 34, spaced plates and 52 have interposedtherebetweena shell 53 to form a reflux feedingchamber 54. A plate 55 ispositioned upon plate 52 and has a plurality of apertures 56 in each ofwhich is secured the open end of a tube 51. Each tube 51 has its top endclosed. Plate 52 is provided with a plurality of apertures 58 each ofwhich supports the lower end of a proportioning tube 59. The arrangementis such that each proportionating tube 59 extends upwardly into aseparate tube 51.

Plate 52 is also provided with a plurality of apertures 6!! of the samenumber as the tubes 51 and 59 as well as the same number asfractionating units 3|.

Depending from each aperture 6!) is a reflux drain tube 6| which extendsdownwardly through a separate aperture 62 in plate 5| and into aseparate fractionating unit 3 I.

It will be understood that any other construction might be substituted.

The operation of the form of the invention shown in Figure 2 is asfollows:

Vapors from any source, such as a still-pot or reboiler, enter chamber45 and are proportioned between the various fractionating units 3|. Ifthe fractionating units 3| are substantially matched as to pressuredrop, the vapors will divide substantially equally between the variousfractionating units. According to the theory under which multi-tubularcolumns operate this is a desired feature, particularly if as is usualthe fractionating units are of similar construction.

The proportioning of the vapors between the various fractionating units3| may, of course, be accomplished otherwise, such as by meteringdevices or as might be the case, depending upon the efficiency desired,this feature might be neg lected.

The vapors to be fractionated ascend through the fractionating units 3|and are contacted therein by descending liquid phase reflux, andrectified vapors escape from the tops 36 of fractionating units 3|.

The rectified vapors then descend through space 33 and thus enclose thefractionating units 3|, flow ofi through opening 44 to condenser 42wherein in the apparatus as illustrated, they are totally condensed.

Condensate flow from outlet 4| of condenser 42 to proportioning pump 40from which a part may be taken off through line 64 as product and therest returned through line 39 to the reflux proportioning cap 34.

Liquid reflux enters chamber 54 wherein it is divided by flowing upthrough proportioning tubes 59 contained within the tubes 51. The refluxliquid is then conducted downward through tubes 5| into the tops offractionating units 3|.

If desired, tubes 6| may be provided at the lower end with cups so as toform a liquid seal to prevent vapors from ascending through these tubes.The cups enable liquid to flow out of tubes 5| into fractionating units3| but prevent the inflow of vapors.

The liquid reflux flows downwardly through fractionating units 3| and iscontacted therein by the ascending vapors.

The liquid reflux leaving the bottoms of fractionating units 3| collectsin chamber 45 and is withdrawn therefrom either continuously,contnually, intermittently, or otherwise, and conducted to a desiredpoint (not shown) such as back to the still-pot or reboiler.

A vent is shown at at the top of condenser 42 which may open to theatmosphere or to which pressure regulating mechanism might be attachedfor operation at sub-atmospheric, atmospheric, or super-atmosphericpressure, as desired.

Any condensate formed in space 33 drains through opening 44 into thebottom 4| of condenser 42.

As in the case of Figure 1, the particular con struction illustrated isfor the purposes of illustration and the column illustrated may bemodifled to adapt it to any type of fractionation. Furthermore, anyother type, form, configuration, or construction might be substituted.

The vapors descending through space 33 minimize the heat gradientbetween the fractionating units 3| and their immediate exterior.

Furthermore, by having the space 33 common to all the fractionatingunits 3|, there is a tendency to maintain the exterior temperature ofeach of the fractionating units 3| the same which is of importance whenit is desired to make very fine or close separations.

If desired, however, for any reason the construction might be modifiedso as to conduct the rectified vapors from. each individualfractionating unit 3| down around that particular fractionating unit.This might be accomplished, for example, by providing each fractionatingunit 3| with a separate casing with its top end closed and .ts bottomend open to the space 33.

The use of the fractionated vapors to enclose or insulate thermally thetubes 3| in a multitubular fractionating column is especially important,for without such adequate insulation, the outermost row or ring of tubes3| is subjected primarily to the loss of heat, but in so doing theyprotect the inner tubes from heat loss and make conditions thereinvirtually adiabatic. But this heat loss from the outer row of tubesupsets their fractionating ability and causes uneven distributionbetween the vapor and liquid quantities flowing in all such tubes 3|.The use of the fractionated vapors in space 33 to jacket the tubesavoids this heat loss from the outer tube row or ring, and verymaterially increases the smoothness and efficiency of operation of allsuch multitubular fractionating devices. These benefits are strictly inaddition to the others occurring as already outlined. The above arepeculiar to multitubular columns.

The construction shown in Figure 2 for multitubular columns isespecially desirable. for complex fabrication problems are avoided. Theproblems of the uneven expansion of the tubes are also eliminated, forthe tubes are all free and not confined in a tube sheet at their upperends. The tubes may now be light weight in construction for theoutermost shell 32 may be designed to withstand any pressure, corrosion,or temperature conditions, instead of requiring each tube 3| to meetsuch conditions. In other words, there is now no material temperature orpressure stress between the inside and outside of tubes 3|.

Many other modifications will occur to persons skilled in the art uponbecoming familiar herewith. For example, it may be desirable to applysome heat insulating material, such as heat resistant tape or sleeving,to the outside of the tubes to reduce exchange of heat between theinside of the tubes and the vapors in space 33. This appliesparticularly if tubes 3| are relatively small in diameter, say 1 inch orless.

As an example of the efficacy of the foregoing methods for renderingadiabatic the fractionating section of distillation columns, thefollowing results are given relating to a 7-inch diameter multitubecolumn, 30 feet tall. This column has the equivalent of '75 to 80theoretical plates. When fractionating gasoline hydrocarbons boilingabout 150 C. to 200 C., the heat loss from the well-insulated column isabout 8000 B. t. u. per hour. In order that the column operate at itsmaximum throughput and efficiency, electrical energy equivalent to this8000 B. t. u. per hour heat loss needs to be supplied to the column bymeans of an electrical resistance winding. The throughput, or amount ofvapors flowing through the column, may be measured from the B. t. u.picked up by the condenser. In operation at about 150 C. about 12,000 B.t. u. per hour are absorbed in the condenser by condensing vapors. Thesevapors are generated in an electrically heated still. This liquidcondensate is collected, and part is returned to the column as refiuxwhile the remainder comprises the product. By providing the column witha jacket through which the vapors may be conducted on their way to thecondenser, the need for the additional 8000 B. t. u. per hour iseliminated. Consequently, the heating load, or the heat requirement ofthe fractionating apparatus, was reduced to about 60 per cent of itsformer value, and the electrical heating of the column may be dispensedwith, by so doing.

In general, my method of maintaining distillation columns substantiallyadiabatic is increasingly desirable the greater the potential heat lossfrom the column, the greater the mass of vapor flowing therethrough, andthe more difficult the fractional distillation being conducted in thecolumn.

The invention is particularly useful as affording an additional tool foruse in solving the problem of separating crude petroleum, or fractionsthereof, into their constituents. Although crude petroleum, for example,has a very wide boiling range and contains materials of a wide varietyof vapor pressures ranging from relativel high to relatively low, yet,apparentl because of the presence of such a large number of differenthydrocarbons, there appears to be no hydrocarbon whose vapor pressure isdistinct. As a result separations and positive identifications have beenlimited'for the most part to the lower molecular weight hydrocarbonspresent.

By applying my invention to the already highly efiicient fractionatingcolumns employed in this work separations previously made can be moreeasily effected and the field for future work considerably extended.

From the foregoing it will be seen that I have provided a highlyefficient method and apparatus for minimizing the heat gradient betweenfractionating columns or units and their immediate exterior, and while Ihave described the invention in connection with given types of columns,it is to be understood that it may be. adapted to anytype of columnregardless of its construction; This includes columns which are built insections, or otherwise.

Having described my invention, it is to be understood that changes,omissions, additions, substitutions and/0r modifications might be madewithin the scope of the claims without departing from the spiritthereof.

I claim:

1. In a process for the fractionation of petroleum hydrocarbons in whichan ascending vapor phase is intimately and countercurrently contactedwith a descending liquid phase in a fractionating zone for purposes offractionation, the step of bringing rectified vapors produced in saidfractionating zone into heat exchange relationship with substantiallythe entire outside circumferential walls of said fractionating zone tosubstantially reduce the heat gradient between said fractiona'ting zoneand its immediate exterior.

2. In a process for the fractionation of a mixture having components ofdifferent volatilities in which an ascending vapor phase is intimatelyand counter-currently contacted with a descending liquid phase forpurposes of fractionation, the steps of forming and maintaining saidphases in a plurality of separate streams, contacting each stream of onephase with a separate stream of the other phase in a separate phasecontacting path, and bringing the rectified vapors produced by saidphase contacting paths into heat exchange relationship circumferentiallyWith substantially the entire outsidesof said phase contacting paths tosubstantially reduce the heat gradient between said phase contactingpaths and their immediate surroundings.

3. In a process for the fractionation of a mixture having components ofdifferent volatilities in which an ascending vapor phase is intimatelyand counter-currently contacted with a descending liquid phase in aphase contacting path for purposes of fractionation, the step of bringinthe rectified vapors produced by said phase contacting path into heatexchange relationship with substantially the entire outsidecircumferential walls of said phase contactin path to substantiallyreduce the heat gradient between said phase contacting path and itsimmediate surroundings.

4. A fractionating column for the fractionation of a mixture havingcomponents of different volatilities in which a vapor phase iscountercurrently contacted with a liquid phase, comprising means forforming and maintaining said phases in a plurality of separate streams,means within said column for contacting each stream of one phase With aseparate stream of the other phase in a separate phase contacting path,and means for bringing the rectified vapors produced by said phasecontacting paths into heat exchange relationship with the outsides ofsaid phase contacting paths. r

5. A fractionating column for the fractionation of a mixture havingcomponents of different volatilities in which a vapor phase iscounter-currently contacted with a liquid phase, comprising a pluralityof phase contacting paths, and means for bringing rectified vaporsproduced by said phase contacting paths into heat exchang relationshipwith the outside of said phase contacting paths to substantially reducethe heat gradient between said phase contacting paths and the immediatesurroundings.

6. A fractionating column comprising a plurality of spaced tubulariractionating zones, means for fixedly supporting each of said tubularfractionating zones at one point thereby leaving said fractionatingzones free from fixed support at other points, means for encasing saidfractionating zones, means for delivering vapors to be fractionated tothe lower ends of said fractionating zones, means for delivering liquidphase reflux to the tops of said fractionating zones, means forconducting rectified vapors into said encasing means adjacent the upperends of said fractionating zones, and means for withdrawing rectifiedvapors from said encasing means adjacent the lower ends of saidfractionating zones.

7. A iractionating column comprising a plurality of spaced tubularfractionating zones, means for fixedly supporting each of said tubularfractionating zones solely adjacent the lower end thereof therebyleaving said fractionatin zones free from fixed support at the upperends thereof, means for encasing said iractionating zones, means fordelivering vapors to be fractionated to the lower ends of saidfractionating zones, means for delivering liquid phase reflux to thetops of said fractionating zones, means for conducting rectified vaporsinto said encasing means adjacent the upper ends of said fractionatingzones, and means for withdrawing rectified vapors from said encasingmeans adjacent the lower ends of said fractionating zones.

8. In a process -for the fractionation of a mixture having components ofdifferent volatilities in which an ascending vapor phase is intimatelyand countercurrently contacted with a descending liquid phase forpurposes of fractionation, the steps of forming and maintaining saidphases in a plurality of separate streams, contacting each stream of onephase with a, separate stream of the other phase in a separate phasecontacting path, combining the rectified vapors produced by said phasecontacting paths, and bringing said combined rectified vapors into heatexchange relationship with the outsides of said phase contacting pathswithin an enclosure common to all of said paths to substantially reducethe heat gradient between said phase contacting paths and the exterior.

MERRELL R. FENSKE.

